(1) Field of the Invention
This invention relates to power supply, particularly to a floating power supply for a voltage follower operating from a high voltage source.
(2) Brief Description of the Related Prior Art
A voltage follower usually uses an operational amplifier (OPA) with the input signal voltage applied to the non-inverting input of the OPA and the output of the OPA connected to the inverting input of the OPA as shown in FIG. 1. Most integrated operational amplifier operates from a low voltage power supply. When the power supply is derived from a high voltage source, the high voltage source must be shifted down to a lower supply voltage for operating the OPA.
A prior art circuit for stepping down the supply voltage in operating the OPA 10 as a voltage follower is shown in FIG. 1. The input voltage Vi is applied to the non-inverting input terminal of the OPA 10. The output of the OPA 10 is connected to a complementary emitter follower with an npn transistor 122 and a pnp transistor 124. The collector of the npn transistor 122 is connected to a high voltage 120V supply, and the collector of the pnp transistor 124 is connected to a high negative voltage -120V supply. The output voltage Vo is connected the common emitter of the npn transistor 122 and pnp transistor 124. The base of the npn transistor 122 is clamped to the output voltage Vo through a Zener diode 147 which is biased by the high positive voltage +120V power source through the resistor 162. Thus, the base of the npn transistor is biased with a voltage equal to Vi+Vz. The positive supply voltage V+ applied to the OPA 10 is then equal Vi+Vz-Vbe, which is a much lower voltage than the 120V high voltage source, where Vbe is the dc base-to-emitter typically around 0.7 V. Similarly, the negative supply voltage V- applied to the OPA 10 is equal Vi-Vz-Vbe, which is much lower in magnitude than the negative -120V high voltage supply.
Since the collector of the npn transistor 122 is connected to the 120V high voltage supply and if Vo centers around 0V, then the V.sub.CE across the npn transistor 122 is around 120V and requires a high voltage npn transistor. Similarly, the large V.sub.CE across the pnp transistor 124 requires a high voltage pnp transistor. Unfortunately, high voltage pnp transistors are not as readily available commercially as npn transistors. It is desirable not to use any high voltage pnp transistors. Another drawback is that the high voltage npn transistor 122 and the high voltage pnp transistor 124 should have matching characteristic. While it is easy to find matched npn transistors, it is difficult to find such a matched complementary pair.